Method for preparing rare-earth halide blocks

ABSTRACT

The invention relates to a method of preparing a polycrystalline block of a halide of formula A e Ln f X (3f+e)  in which Ln represents one or more rare earths, X represents one or more halogen atoms selected from the group consisting of Cl, Br and I, and A represents one or more alkali metals selected from the group consisting of K, Li, Na, Rb and Cs, e, which may be zero, being less than or equal to 3 f , and  f  being greater than or equal to 1, having a low water and oxyhalide content, in which the method comprises heating a mixture of, on the one hand, at least one compound having at least one Ln—X bond and, on the other hand, a sufficient amount of NH 4 X in order to obtain the oxyhalide content, resulting in a molten mass comprising the rare-earth halide, the heating being followed by cooling, and the heating, after having reached 300° C., never going below 200° C. before the molten mass has been obtained. The blocks thus produced allow very pure single crystals having remarkable scintillation properties to be grown.

The present application is a national stage application of InternationalApplication No. PCT/FR03/03356, filed on Nov. 13, 2003, which publishedas WO 04/050792 on Jun. 17, 2004, and claims priority to FrenchApplication No. 02/14856, filed on Nov. 27, 2002, the entire contents ofwhich are hereby incorporated by reference.

Rare-earth halides (in what follows, Ln is used to denote a rare earth),especially when they are doped with cerium, and in particularcerium-doped LnBr₃ and cerium-doped LnCl₃, have very usefulscintillation properties especially for applications in nuclear imagingand in spectroscopy (positron emission tomography or PET, gamma camera,oil prospecting and the like). To obtain these propertiessatisfactorily, it is necessary for these compounds to be obtained inthe form of large crystals. Generally, these crystals are singlecrystals. In certain particular cases, they may be polycrystals, withinwhich the crystals have one dimension of the order of one or morecentimeters. However, rare earth halides are highly hygroscopiccompounds that react with water and with air as soon as they are heated,forming very stable oxyhalides. It has in general been considered thatoxyhalide contents of the order of 0.1% by weight were acceptable, thecrystals obtained with these contents being sufficiently transparent inappearance. In addition, certain crystals, such as CsI:TI, accommodatehigh oxygen contents (for example around 0.2% of CsOH) as far as thescintillation properties are concerned. Now, the Applicant hasdiscovered that the scintillation properties, especially the luminousefficiency, that is to say the number of UV-visible photons emitted perMeV of energy of an incident particle, of rare-earth halides can bedrastically improved by lowering the oxyhalide content in a rare-earthhalide crystal below this value.

The Applicant therefore sought to develop manufacturing methods thatresult in rare-earth halides that are as pure as possible (especially asregards oxygen), that is to say the water content of which is very muchless than 0.1% by weight and the oxyhalide content of which is less than0.2% by weight, and even less than 0.1% by weight or indeed less than0.05% by weight. Moreover, means have to be found for preserving (forexample over several months) and handling these halides that allow thispurity to be maintained. This is because the growth of the crystals(generally single crystals) is usually carried out in batch mode, whichinvolves phases of putting them into storage and of removing them fromstorage, which phases are conducive to contamination of the rare-earthhalide by the water and oxygen of air.

In addition, it is very difficult to produce an installation forpreparing a rare-earth halide (as raw material for growing crystals,generally single crystals) that does not itself introduce a smallquantity of water or oxygen resulting in the formation of an undesirableoxyhalide. This is because any installation is always imperfectlyimpermeable and also always contains a small quantity of adsorbed water,so that partial contamination is usual in this kind of preparation, anda high degree of oxidation by the impurities of the gaseous environmentis generally expected, most particularly at high temperatures such asabove 300° C. The invention also provides a solution from thisstandpoint since the method according to the invention results in a verypure rare-earth halide, even with an installation initially containingwater, whether adsorbed, absorbed or in condensed phase, and even in thepresence of a reasonable amount of water and oxygen in the atmosphereduring the heating leading to melting.

The Applicant has discovered that crystals manufactured according to theinvention can even have melting points substantially different fromthose mentioned in the literature, this being interpreted as an effectowing to the high purity of the crystals (especially a low oxychloridecontent) obtained thanks to the invention. Thus, an LaCl₃ crystalproduced according to the invention has a crystallization temperature of880° C., whereas the values published by the prior art are spreadbetween 852 and 860° C. Likewise, an LaBr₃ crystal manufacturedaccording to the invention has a crystallization temperature of 820° C.,whereas the values published in the prior art are between 777° C. and789° C.

The invention especially makes it possible to prepare single crystalshaving a particularly low scintillation decay time. The advantage ofthis is that it is desirable to have crystals whose scintillation peakshave the lowest possible decay time, as in this way the time resolutionis improved. To make this measurement, the light intensity of the mainpeak is recorded over time. Thus, the invention allows the production ofsingle crystals whose decay time of the main component is less than 40,and even less than 30 and even less than 20 nanoseconds. For the purposeof the present invention, X represents a halogen atom chosen from Cl, Brand I. The present invention does not relate to rare-earth fluoridesbecause they are not hygroscopic and because their chemistry is highlyspecific.

The single crystals prepared according to the invention also have aparticularly low energy resolution, especially less than 5%, or evenless than 4% or even less than 3.5%.

The following conventional methods may be used to prepare a rare-earthhalide:

1. vacuum dehydration of LaX₃(H₂O)₇ at 80° C.: but this method givesLaOX contents that are too high and results in crystals of low quality;

2. chlorination of solid La₂O₃ with HCl gas above 500° C.: this methodis dangerous as it requires the use of large amounts of HCl gas—a toxicgas—and it is also very difficult to ensure on an industrial scale thatthe chlorination reaction is complete;

3. dehydration of LaX₃(H₂O)₇ in gaseous HX. This method is alsodangerous because of the large amount of HX used; and

4. reaction of an La₂O₃ powder with gaseous NH₄Cl at about 340° C.: thisis very difficult to ensure on an industrial scale that the chlorinationreaction is complete.

The document “The ammonium-bromide route to anhydrous rare earthbromides MBr ₃”; Journal of the Less-Common Metals, 127 (1987) 155-160teaches the preparation of a rare-earth halide/ammonium bromide complexand its thermal decomposition at less than 20° C./hour in order to forma rare-earth halide, without ever reaching the melting point. Byoperating in this way, the halide retains a high specific surfacearea—greater than 0.1 m²/g—propitious to moisture absorption and tooxychloride formation. The fact of working at below 400° C. greatlylimits the problems of corrosion of materials, and this is one of thereasons why it is preferred in the prior art to use such lowtemperatures. The prior art making use of compounds of the NH₄X typedoes not generally heat beyond 300 or 400° C., as at higher temperaturesNH₄X disappears by sublimation and the rare-earth halide becomesparticularly sensitive to oxidation from the traces of water and oxygenthat are present in the gaseous environment.

As documents of the prior art, mention may also be made of WO 0160944,WO0160945 and U.S. Pat. No. 6,451,106.

The invention solves the abovementioned problems. The invention makes itpossible to obtain a very pure rare-earth halide in the form of apolycrystalline block, especially one having a rare-earth oxyhalidecontent of less than 0.2% by weight, or even less than 0.1% by weight,or even less than 0.05% by weight or even less than 0.02% by weight, anda water content of less than 0.1% by weight.

The method of preparation according to the invention comprises a step ofheating a mixture of, on the one hand, at least one compound having atleast one bond and, on the other hand, NH₄X, in which Ln represents arare earth and X is chosen from Cl, Br and I, said compound and NH₄Xpossibly being combined, at least partially, within a complex, said stepresulting in a molten phase comprising the intended halide, followed bya cooling step resulting in at least one solid block comprising saidhalide. The NH₄X, by reacting with the oxyhalides, acts as oxygenscavenger and consequently strips the rare-earth halide of itsoxychlorides, given that these oxychlorides may come from the reactionbetween the water that is absorbed by the rare-earth halide and therare-earth halide during the heating. This purification takes placeaccording to the principle of the following reaction:LnOX+2NH₄X→LnX₃+H₂O+2NH₃

The method according to the invention makes it possible in particular toprevent water, present in the mixture or the crucible or the apparatus,in adsorbed, absorbed or complexed form, from combining permanently withthe rare-earth halide into the rare-earth oxychloride. Thus, the methodaccording to the invention results in a final block having much lessoxyhalide than the same method without the initial NH₄X. In particular,this may be observed with an installation difficult to strip of itsadsorbed water, that is to say an installation that usually results in ahigh oxyhalide content (for example at least 0.2% oxyhalide) in thefinal halide even when no oxyhalide is intentionally put into thestarting mixture (or a very low content, i.e. less than 100 ppm byweight) and even in the presence of the usual water and oxygen contentsin the gaseous environment for this kind of manufacture.

The polycrystalline block obtained according to the invention is verypure. The invention combines, in a single heating step, the oxygenscavenging action, conferred by the presence of the ammonium halide, andthe fact of immediately proceeding to melting the rare-earth halide soas to drastically reduce its specific surface area, thereby making itless moisture-sensitive while it is being stored and handled. The halideis therefore purified in a first stage and then melted in a second stageso as to become much less sensitive to oxidation by water and oxygen,these first and second stages being carried out within one and the sameheating step, which means that, once the mixture has reached thetemperature of 300° C., its temperature does not return to roomtemperature or even to a temperature below 200° C. before the desiredrare-earth halide has been melted. This preparation of the blockaccording to the invention is carried out in an inert or neutralatmosphere (for example in nitrogen or argon), but this atmosphere mayeven contain relatively large amounts of water and oxygen, that is tosay in such a way that the sum of the water and oxygen masses in thegaseous atmosphere is less than 200 ppm by weight. In general, duringproduction of the block according to the invention the water content ofthe inert atmosphere ranges from 10 to 180 ppm by weight and the oxygencontent of the atmosphere ranges from 0.5 to 2 ppm by weight.

Because of its low specific surface area compared with a powder, theblock absorbs fewer impurities from the air (moisture and oxygen) andconsequently can be stored and handled while retaining a very high purestate. Under these conditions, this block can be used for preparingcrystals (generally single crystals) of rare-earth halides that are verypure and of high quality.

The invention also relates to a method of preparing the blocks accordingto the invention in a carbon-rich crucible. Whereas according to theprior art, such as for example the article by P. Egger et al., J.Crystal Growth 200 (1999) 515-520, the growth of Ba₂Y_(1−x)Er_(x)Cl₇(0<x<1) in a glassy carbon crucible leads to contamination of thecrystal because of the crucible, it turns out that the compositionsforming the subject of the present invention are advantageously meltedin a crucible that is rich in carbon, as is the case with glassy carbon,in order to produce the block according to the invention.

The rare earths Ln to which the present invention relates are those ofColumn 3 (according to the new notation) of the Periodic Table of theElements, including Sc, Y, La, and the lanthanides from Ce to Lu. Moreparticularly involved are the halides of Y, La, Gd and Lu, which mayespecially be doped with Ce or Pr.

The rare-earth halides more particularly involved in being manufacturedin block form according to the present invention may be represented bythe general formula A_(e)Ln_(f)X_((3f+e)) in which Ln represents one ormore rare earths, X represents one or more halogen atoms chosen from Cl,Br or I, and A represents one or more alkali metals such as K, Li, Na,Rb or Cs, e and f representing values such that:

-   -   e, which may be zero, is less than or equal to 3f;    -   f is greater than or equal to 1.

The method according to the invention is more effective the lower theatomic number of X. Thus, the effectiveness of the method according tothe invention in reducing the oxyhalide content in the final blockincreases, depending on the nature of X, in the following order:I<Br<Cl. The method according to the invention is more effective thelarger the ionic radius of Ln. Thus, the effectiveness of the methodaccording to the invention in reducing the oxyhalide content in thefinal block increases, depending on the nature of Ln, in the followingorder: Sc<Lu<Y<Gd<Pr<Ce<La.

The rare-earth halides more particularly involved are especially thefollowing:

-   -   ALn₂X₇ in which Ln represents one or more rare earths, X        represents one or more halogen atoms chosen from Cl, Br or I, A        representing an alkali metal such as Rb and Cs;    -   LaCl₃, which may especially be doped with 0.1 to 50% by weight        of CeCl₃;    -   LnBr₃, which may especially be doped with 0.1 to 50% by weight        of CeBr₃;    -   LaBr₃, which may especially be doped with 0.1 to 50% by weight        of CeBr₃;    -   GdBr₃, which may especially be doped with 0.1 to 50% by weight        of CeBr₃;    -   La_(x)Ln_((1−x))X₃, which may especially be doped with 0.1 to        50% of CeX₃, it being possible for x to range from 0 to 1, Ln        being a rare earth different from La, X being a halogen as        mentioned above;    -   La_(x)Gd_((1−x))Br₃, which may especially be doped with 0.1 to        50% of CeBr₃, it being possible for x to range from 0 to 1;    -   La_(x)Lu_((1−x))Br₃, which may especially be doped with 0.1 to        50% of CeBr₃, it being possible for x to range from 0 to 1;    -   Ln′_(x)Ln″_((1−x))X′_(3(1−y))X″_(3y) in which Ln′ and Ln″ are        two different rare earths of the Ln type, X′ and X″ being two        different halogens of the X type, especially Cl and Br, it being        possible for x to range from 0 to 1, and y to range from 0 to 1;    -   RbGd₂Br₇, which may especially be doped with 0.1 to 50% by        weight of CeBr₃;    -   RbLn₂Cl₇, which may especially be doped with 0.1 to 50% by        weight of CeCl₃;    -   RbLn₂Br₇, which may especially be doped with 0.1 to 50% by        weight of CeBr₃;    -   CsLn₂Cl₇, which may especially be doped with 0.1 to 50% by        weight of de CeCl₃;    -   CsLn₂Br₇, which may especially be doped with 0.1 to 50% by        weight of CeBr₃;    -   K₂LaCl₅, which may especially be doped with 0.1 to 50% by weight        of CeCl₃;    -   K₂Lal₅, which may especially be doped with 0.1 to 50% by weight        of CeI₃.

The term “dopant” or “doped” refers to a rare earth present in a minorquantity that substitutes for one or more rare earths in a majorquantity, the rare earths in a minor and major quantity both beingdenoted by the symbol Ln.

Thus, the invention may especially result in a block in which Ln is Laor Ce and X is Cl or Br.

The invention relates especially to a method of preparing a blockcharacterized in that it comprises a step of heating a mixture of, onthe one hand, at least one compound having at least one Ln—X bond and,on the other hand, NH₄X, said compound and NH₄X possibly being combined,at least partially, within a complex, said step resulting in a moltenmass comprising the rare-earth halide of formula A_(e)Ln_(f)X_((3f+e)),said heating step being followed by a cooling step after the molten masshas been obtained, and said heating step, after having reached 300° C.never going back down below 200° C. before said molten mass has beenobtained.

The compound having at least one Ln—X bond may be of formulaA_(r)Ln_(s)O_(u)X_(r+3s−2u) in which A, X and Ln have the meanings givenabove, r, s and u representing integer or noninteger values that meet,cumulatively, the following conditions:

r ranging from 0 to 2s,

s being greater than or equal to 1,

u ranging from 0 to s,

which compound may or may not be complexed with water or with NH₄X.

In the compound having at least one Ln—X bond, Ln is in oxidation state3 and, if A is present, this is in oxidation state 1. In particular, rmay be zero. In particular, u may be zero.

Preferably, in the compound having at least one Ln—X bond, the amount ofoxygen bonded to Ln is such that the amount of oxyhalide obtained by thedissolution method is less than 100 ppm by weight.

The compound having at least one Ln—X bond may be a rare-earth halide ora hydrated rare-earth halide. For example, it may be of formula LnX₃ orLnX₃.(H₂O)_(n) with n ranging from 1 to 10, or a mixture of several ofthe compounds whose formulae have just been given.

The compound having at least one Ln—X bond may also be a rare-earthoxyhalide. It may be of formula LnXO or a mixture of several of thecompounds whose formulae have just been given. It is preferable to avoidthe presence of LnXO in the starting mixture. Thus, preferably, thestarting mixture contains less than 100 ppm by weight of Ln-XO. Ingeneral, this is a rare-earth halide in powder form containing a smallproportion of oxyhalide and water. The mixture may also include arare-earth oxyhalide/NH₄X complex.

The mixture may also contain water, either in free form or in complexedform, for example complexed with the rare-earth halide. Surprisingly,the amount of water may be very high without this resulting in a higheroxyhalide content being obtained in the final polycrystalline blockaccording to the invention, as long as the mixture contains a sufficientamount of NH₄X.

The mixture may even include, for example, up to 20% by weight of water,or more. It may also include, for example, less than 16% by weight ofwater, or even less than 5% by weight of water.

The mixture of, one the one hand, at least one compound having at leastone Ln—X bond and, on the other hand, NH₄X, these two compounds being,where appropriate, at least partially in complexed form, containssufficient NH₄X for the final block to comprise less than 0.2% by weightof rare-earth oxyhalide, or even less than 0.1% by weight of rare-earthoxyhalide, or even less than 0.05% by weight of rare-earth oxyhalide, oreven less than 0.02% by weight of rare-earth oxyhalide.

Preferably, the Ln atoms in the compound are linked only to X atoms oroxygen atoms or A atoms. This is particularly the case for a complex offormula LnX₃.(NH₄X)_(x) in which the Ln atoms are linked only to Xatoms. Here, it is considered that no atom belonging to NH₄X is linkedto the Ln atoms.

Preferably, an amount of NH₄X which is at least the sum of the followingtwo quantities:

-   -   A) a number of moles of NH₄X equal to one and preferably three        times the number of moles of Ln that are not linked to an        oxygen;    -   B) is a number of moles of NH₄X equal to three times and        preferably five times the number of moles of oxygen atoms linked        to Ln, is introduced into the mixture.

In particular, it is possible to introduce, into the mixture, an amountof NH₄X which is at least the sum of the two following quantities:

-   -   A) a number of moles of NH₄X equal to three times the number of        moles of Ln that are not linked to an oxygen;    -   B) a number of moles of NH₄X equal to five times the number of        moles of oxygen atoms linked to Ln.

It is clearly understood that if NH₄X is counted for the purpose ofcalculating A), the same NH₄X must not be counted for the purpose ofcalculating B), and vice versa. If the mixture contains no oxygen linkedto Ln, the amount of NH₄X in the case of B) by itself is zero.

For the purpose of the present invention, it is considered that thenumber of moles of oxygen atoms linked to Ln is identical to the numberof moles of oxyhalide of formula LnOX as obtained by the dissolutionmethod described below. It is therefore easy to calculate, from the massof oxyhalide obtained by the dissolution method, the number of moles ofoxygen atoms linked to Ln by assuming that the oxyhalide has the formulaLnOX. If A (generally Rb or Cs) is present, because this atom has a verylow tendency to combine with oxygen, its presence is not involved in thecalculations of the amounts of NH₄X.

It will be clearly understood that, to calculate this amount, it isnecessary to take account of all the NH₄X molecules present in themixture, whether or not this NH₄X is complexed, for example with arare-earth halide.

The mixture may comprise a mixture of the compound having at least onebond and NH₄X. This complex may, for example, be prepared by wetchemistry, according to the following principle:

-   -   a rare-earth salt, such as a rare-earth oxide or a hydrated        rare-earth halide is firstly dissolved in the corresponding        hydroacid (that is to say HCl if it is desired to obtain a        chloride, HBr if it is desired to obtain a bromide). At this        stage, AX (A generally being Rb or Cs) is added if a halide        containing A is desired. Preferably, added to the ammonium        halide solution are 1 to 4 moles of ammonium halide per mole of        rare-earth halide, so as to obtain a solution. If it is desired        finally to obtain a halide of the rare-earth Ln doped with        another rare earth Ln′ (in fact doped with a halide of Ln) such        as cerium, all that is required is to introduce, during        dissolution in the hydroacid, the desired proportion of Ln′ (for        example, 10% of CeX₃(H₂O)₇ into a solution obtained from        LaX₃(H₂O)₇ if it is desired finally to obtain anhydrous LnX₃        doped with 10% of CeX₃).    -   The solution is then dried in an oven or by any suitable means.        The salt obtained, of formula LnX₃. (NH₄X)_(x), where x=3.5, is        stable and can be stored in sealed containers.

The mixture of, on the one hand, at least one compound having at leastone bond and, on the other hand, NH₄X is then subjected to a heattreatment. For this heat treatment, the mixture is generally placed in acrucible, which may be made of platinum, carbon, such as graphite, ormolybdenum or tantalum or boron nitride or silica. The crucible may alsobe made of graphite coated with pyrolytic carbon or made of graphitecoated with silicon carbide or made of graphite coated with boronnitride. Preferably, for the melting, a crucible allowing the block tobe demolded cold is used. To produce the blocks according to theinvention, it is preferable to use a crucible made of a materialcontaining at least 20% carbon by weight. Such a material may, forexample, be carbon or graphite, or amorphous carbon (or glassy carbon)or graphite coated with pyrolytic carbon (also glassy carbon) orgraphite coated with silicon carbide, or graphite coated with boronnitride (possibly pyrolytic). The crucible may therefore be coated witha layer of pyrolytic carbon. The material may comprise, on the one hand,a graphite substrate and, on the other hand, a coating, it beingpossible for this coating to be made of pyrolytic carbon or of siliconcarbide or of boron nitride (possibly pyrolytic). The coating servesespecially to block the pores in the graphite.

The crucible is then positioned in a sealed furnace, the atmosphere fromwhich is purged in order to render it inert, for example purged under alow vacuum and then flushed with a stream of dry nitrogen. Thetemperature of the furnace is then progressively raised up to at least400° C. The water from the complex is eliminated and then NH₄X sublimesand is deposited on the cool downstream parts of the furnace. It isimportant for the mixture to be protected from the ambient air and to becompletely in an inert atmosphere, especially above 300° C. andpreferably above 200° C. This is why the potential air intakes into theinstallation are located beyond the point where the NH₄X is deposited sothat air cannot get back into the mixture being purified.

Owing to the fact that the NH₄X is generally present in excess in themixture, the actual temperature of the mixture generally presents atemperature hold corresponding to the temperature at which the NH₄X iseliminated, even if the programmed temperature is constantly increasing.In the case of NH₄Cl, this temperature hold lies between 300 and 400° C.This applies not only if the NH₄X is initially in free form but also ifit is in complexed form. Because the heated mass contains much less NH₄Xafter this temperature hold, it might be expected that the mixture wouldthen be easily oxidized by the impurities present in the gaseousenvironment (presence of water and oxygen), and to be more so the highthe temperatures (at this stage, the temperature of the heated mass isgenerally above 300° C.). The Applicant has discovered that this is notthe case and that it is possible to control the oxidation of therare-earth halide.

After the temperature hold during which the NH₄X is given off, thetemperature must then be rapidly increased up to a temperaturesufficient to melt the desired rare-earth halide (for example, 880° C.in the case of LaCl₃). After the temperature hold at which the NH₄X isgiven off, generally between 300 and 400° C., the mixture, alreadyconverted compared with the starting material (since it has lost theNH₄X), may be heated at a rate of greater than 50° C./hour and evengreater than 100° C./hour and even greater than 150° C./hour and evengreater than 200° C./hour. In general, the heating rate is less than600° C./hour owing to the fact that it is generally necessary to protectthe materials of the installation according to their thermal-shockresistance. When the heated material is molten, it is preferred tomaintain a temperature above the melting point for at least one hour,generally between one and six hours.

As regards heating the mixture, once this is at a temperature above 300°C., its temperature is not brought back down to room temperature, oreven to a temperature below 200° C., before the desired rare-earthhalide has reached the melting point. It is preferable to heat themixture until melting in a single heating step, without lowering thetemperature, even momentarily, before the molten mass comprising themolten halide is obtained. The entire heating step (from roomtemperature up to melting) may generally be carried out in less than 10hours, or even less than 6 hours or even less than 4 hours.

The molten mass can then be rapidly cooled. A block of anhydrousrare-earth halide is thus recovered, this comprising less than 0.1% byweight of water and less than 0.2% by weight of rare-earth oxyhalide, oreven less than 0.1% by weight of rare-earth oxyhalide, or even less than0.05% by weight of rare-earth oxyhalide or even less than 0.02% byweight of rare-earth oxyhalide. This block is easy to handle and tostore. In general, blocks of at least 1 g per unit, or indeed at least10 g per unit, or indeed at least 50 g per unit or indeed at least 500 gper unit may be produced. These blocks generally have a bulk density ofat least 75%, or indeed at least 80%, or indeed at least 85% of thetheoretical density, it being understood that the theoretical density isthat corresponding to the same material without any porosity. The blockaccording to the invention is polycrystalline and contains a multitudeof grains, each of which is a small single crystal. A block generallycontains at least 100 grains and even at least 1000 grains. No grain ofthe block represents more than 10% of the entire mass of the block.

The ammonium halide condensed on the cool downstream parts of thefurnace may be at least partly reused, for example in the methodaccording to the invention.

To measure the content of oxyhalides in a rare-earth halide, all that isrequired is to separate them using water (for example at roomtemperature) since the oxyhalides are insoluble in water whereas thehalides are soluble. The oxyhalides may be recovered by filtration, forexample over a polypropylene (PP) filter and then dried at 120° C. Ifthe halide includes A (generally Rb or Cs), this method results in thedissolution of AX since A does not form an oxyhalide. This method,called the “dissolution method” of the “method of insolubles”, results,even in the presence of A in the halide, in a determination of thecontent of oxyhalide of formula LnXO.

The block according to the invention may be used as raw material forgrowing crystals (generally single crystals) using known techniques suchas Bridgman growth or Kyropoulos growth or Czochralski growth, or growthusing the gradient freeze method. These single crystals are very pureand can be used as scintillator material. This crystal preparation iscarried out in an inert atmosphere (nitrogen or argon for example), butthis atmosphere may contain relatively large amounts of water andoxygen, that is to say in such a way that the sum of the water andoxygen masses in the gaseous atmosphere is less than 200 ppm by weight.In general, during production of the crystal (generally a singlecrystal), the water content of the inert atmosphere ranges from 10 to180 ppm by weight and the oxygen content of the atmosphere ranges from0.5 to 2 ppm by weight.

Because of the low surface area of the block or blocks used as rawmaterial and also because this surface area increases during the rise intemperature up to melting, the final single crystal is very pure and hasa remarkable scintillation efficiency. Thus, the invention also relatesto a single crystal of formula A_(e)Ln_(f)X_((3f+e)), the symbols ofwhich have the meanings given above, said single crystal comprising lessthan 0.2% and even less than 0.1%, or indeed less than 0.05% or indeedless than 0.02% by weight of rare-earth oxyhalide. This appliesespecially when Ln is chosen from La, Gd, Y, Lu and Ce, and when X ischosen from Cl and Br. Reference may more particularly be made to thefollowing single crystals:

those of general composition Ln_(1−x)Ce_(x)Br₃ in which Ln is chosenfrom lanthanides or mixtures of lanthanides of the group La, Gd, Y andLu, especially chosen from lanthanides or groups of lanthanides of thegroup La and Gd, and in which x is the molar degree of substitution ofLn with cerium, where x is greater than or equal to 0.01 mol % andstrictly less than 100 mol %;

those of general composition Ln_(1−x)Ce_(x)Cl₃, in which Ln is chosenfrom lanthanides or mixtures of lanthanides of the group Y, La, Gd andLu, especially from elements or mixtures of elements of the group La, Gdand Lu, and in which x is the molar degree of substitution of Ln withcerium, where x is greater than or equal to 1 mol % and strictly lessthan 100 mol %.

The abovementioned growth methods may result in a large single crystal,that is to say one at least 1 cm³, or indeed at least 10 cm³ and even atleast 200 cm³ in size. This single crystal may then be cut to the sizessuitable for the desired applications.

The single crystal according to the invention, because of its highpurity, has a particularly high luminous efficiency. One way ofmeasuring this luminous efficiency is to measure it relative to that ofa crystal of NaI doped with 600 ppm by weight of TI iodide, the energyresolution of which at 622 keV is 6.8%, the integration time being 1 μsand the radioactive source being ¹³⁷Cs at 622 keV. The coupling betweenthe crystals (NaI or rare-earth halide) and the photomultiplier takesplace by means of a silicone grease that is transparent up to 320 nm. Ofcourse, the exit face of the NaI turned toward the photomultiplier ispolished. Under these measurement conditions, the invention makes itpossible to obtain luminous efficiencies of at least 90% of that of theTI-NaI crystal, and in any case greater than those obtained on crystalsnot according to the invention.

The crystal or single crystal may especially be produced in a cruciblemade of platinum or graphite or graphite coated with pyrolytic carbon.

In the examples that follow, the energy resolution was measured in thefollowing manner: a 10×10×5 mm piece is cut from the single crystal. Allthe faces of the piece apart from one of the large 10×10 mm faces areleft as cut and the face with which the photomultiplier (PMT) couples ispolished. The crystal is wrapped in several thickness of PTFE (Teflon)tape except on the face that is coupled to the PMT. The crystal isprepared in a glove box, the dew point of which is below −40° C.

EXAMPLE 1 Anhydrous LaCl₃

433 g of La₂O₃ were dissolved by 1380 ml of 37% HCl diluted in 2450 mlof water. 497 g of NH₄Cl were added. Next, the water and HCl in excesswere evaporated by heating to 100° C. in order to obtain anLaCl₃.(NH₄Cl)_(3.5) complex, which contained 0.7% by weight of waterusing the Karl Fischer measurement. The LaCl₃.(NH₄Cl)_(3,5) complex wasa compound that did indeed have at least one bond, since it containedLa—Cl bonds. It was also itself a mixture within the meaning of theinvention, comprising, on the one hand, a compound having an Ln—X bondand, on the other hand, NH₄X (in this case NH₄Cl). Moreover, within thismixture, the amount of NH₄X is such that the ratio of the number ofmoles of NH₄X to the number of moles of Ln not linked to oxygen is 3.5,which corresponds to a preferred ratio according to the invention.Moreover, it was unnecessary to include in our calculations for NH₄Xbeing introduced in the case of oxygen linked to Ln since the startingmixture did not contain this type of bond.

By heating at a rate of 200° C./hour from room temperature up to 950°C., 200 g of the complex were then decomposed with sublimation of theNH₄Cl and melted in a crucible made of graphite coated with pyrolyticcarbon, said crucible itself being placed in a sealed silica tube with anitrogen flush. The nitrogen atmosphere contained about 50 ppm by weightof water and between 1 and 2 ppm oxygen by weight. The oxychloridecontent in the final block, measured by dissolution, was 0.01% byweight. The water content was less than 0.1% by weight (the detectionlimit of the method used). The block obtained had a mass of 651 g.

EXAMPLE 2 (Comparative Example): Anhydrous LaCl₃

The procedure was exactly as in the case of example 1, except that thecomplex was replaced with an anhydrous LaCl₃ powder whose oxychloridecontent was less than 0.02%, the size of the particles wassubmillimetric and the water content was not detectable by Karl Fischer.

The oxychloride content in the final block, measured by dissolution, was0.23% by weight. The water content was less than 0.1% by weight.

EXAMPLE 3 Anhydrous LaBr₃:Ce

300 g of La₂O₃ were dissolved by 630 ml of 47% HBr diluted in 2330 ml ofwater. 682 g of NH₄Br were added. The solution obtained was filteredover PP. The solution was then dried with a rotary evaporator in a 10 Iflask. The complex obtained, of formula LaBr₃.(NH₄Br)_(3.5), contained0.23% water by weight measured using Karl Fischer. Next, 142.6 g of thiscomplex was removed and doped with 0.5% by weight of (NH₄Br)_(3.5)CeBr₃,and this was heated at 200° C./h in a stream of nitrogen in a graphitecrucible. The temperature was held at 860° C. for 4 h 30. The nitrogenatmosphere contained approximately 50 ppm by weight of water and between1 and 2 ppm by weight of oxygen. The melted pellet weighed 76.61 g andcontained only 0.035% oxybromide LaOBr (measured by the method ofinsolubles). The water content was also less than 0.1%.

The hydrostatic density of this block, measured by immersion in hexane,was approximately 4.92 g/cm³, i.e. 87% of the theoretical density,thereby proving good densification.

This melted block was then used for growth in a Bridgman furnace in agraphite crucible under a stream of nitrogen. The nitrogen atmospherecontained approximately 50 ppm by weight of water and between 1 and 2ppm by weight of oxygen. The crystal obtained was clear and free ofwhite oxybromide inclusions and free of bubbles. The oxybromide contentof this crystal was 0.05% by weight. More than 80% of the mass of thiscrystal was suitable for use as a scintillator.

EXAMPLE 4 Anhydrous LaBr₃ from a Wet Complex

The complex LaBr₃.(NH₄Br)_(3.5) prepared as in the preceding example wasused, but wetted so that it contained 14.7% by weight of water measuredby Karl Fischer. 124 g of this mixture (complex+water) were removed andheated at 200° C./h in a stream of nitrogen in a graphite crucible up to860° C. The temperature was held at 860° C. for 4 h 30. The nitrogenatmosphere contained approximately 50 ppm by weight of water and between1 and 2 ppm by weight of oxygen. The melted pellet weighed 64.1 g andcontained only 0.034% by weight of oxybromide (measured by the method ofinsolubles). The water content was less than 0.1% by weight.

EXAMPLE 6 Anhydrous GdBr₃

271.2 g of Gd₂O₃ were dissolved by 796 g of 48% HBr diluted in 430 g ofwater. Next, 661.2 g of NH₄Br and 855 g of water were added. Thesolution obtained was filtered over PP. The solution was then dried in arotary evaporator in a 10 I flask. 1164 g of the complex(NH₄Br)_(4.5)GdBr₃ were then obtained. The complex obtained contained6.3% water measured by Karl Fischer. Next, 254.7 g of this complex wereremoved and heated at 200° C./h in a stream of nitrogen in a graphitecrucible. The temperature was held at 815° C. for 1 h 30. The nitrogenatmosphere contained approximately 50 ppm by weight of water and between1 and 2 ppm by weight of oxygen. The sintered but unmelted pelletweighed 104.9 g. It was therefore a pulverulent solid that was returnedto the ambient conditions. The furnace was recharged with 92.7 g of theabove sintered pellet and this was heated at 200° C./h under a stream ofnitrogen in a graphite crucible. The temperature was held at 840° C. for1 h 30. The nitrogen atmosphere contained approximately 50 ppm by weightof water and between 1 and 2 ppm by weight of oxygen. The melted pelletweighed 92.7 g and contained 0.65% by weight of GdOBr (measured by themethod of insolubles), thereby demonstrating the fact that returning theblock to room temperature before melting is contraindicated.

EXAMPLE 7 Anhydrous GdBr₃

The complex (NH₄Br)_(4.5)GdBr₃ as prepared according to the precedingexample was used for this test. The complex obtained contained 6.3% byweight of water measured by Karl Fischer. Next, 245.7 g of this complexwere removed and heated at 200° C./h in a stream of nitrogen in agraphite crucible up to 840° C. The temperature was held at 840° C. for1 h 30. The nitrogen atmosphere contained approximately 50 ppm by weightof water and between 1 and 2 ppm by weight of oxygen. The melted pelletweighed 105.3 g and contained only 0.038% by weight of oxybromide GdOBr(measured by the method of insolubles). This result is all the moreexemplary owing to the fact that gadolinium is a heavy (so-calledyttric) rare earth whose bromides are very sensitive to hydration.

EXAMPLE 8 (Comparative Example): Single Crystal from LaCl₃ Powder

The same batch of anhydrous LaCl₃ powder was used as that used forexample 2 for Bridgman furnace growth in a graphite crucible in a streamof nitrogen. The nitrogen atmosphere contained approximately 50 ppm byweight of water and between 1 and 2 ppm by weight of oxygen. The crystalobtained had many white oxychloride inclusions and bubbles organized inthe form of filaments along the pulling axis. The oxychloride content ofthis crystal was 0.25% by weight. Approximately 90% of the mass of thiscrystal was unsuitable for use as a scintillator.

EXAMPLE 9 Anhydrous RbGd₂Cl₇

138.2 g of Rb₂CO₃ were dissolved by 242 g of 37% HCl diluted in 165 g ofwater. The solution obtained was filtered over PP. Next, 433.8 g ofGd₂O₃ were dissolved by 750 g of 37% HCl diluted in 482 g of water.After complete dissolution, the filtered rubidium solution was added.Finally, 576.2 g of NH₄Cl and 881 g of water were added. The solutionobtained was filtered over PP. The pH was −0.32 and the density of thesolution was 1.24. The solution was then dried in a rotary evaporator ina 10 I flask. 1227 g of (NH₄Cl)_(g)RbGd₂Cl₇ were then obtained. Next,142.6 g of this complex were removed and heated at 200° C./h in a streamof nitrogen in a graphite crucible up to 660° C. The temperature washeld at 660° C. at for 4 h 30. The nitrogen atmosphere containedapproximately 50 ppm by weight of water and between 1 and 2 ppm byweight of oxygen. The melted pellet contained only less than 0.05% byweight of GdOCl (measured by the method of insolubles).

EXAMPLE 10 Synthesis from LaOBr

The following mixture was produced in a glassy carbon crucible: 0.5874 gof LaOBr, 1.3585 g of NH₄Br (i.e. 5.5 moles) and 10.0678 g of the(NH₄Br)_(3.5)LaBr₃ complex. The mixture was heated at a rate of 200°C./h up to 830° C., with a hold at this temperature for 2 h. Thenitrogen atmosphere contained approximately 50 ppm by weight of waterand between 1 and 2 ppm by weight of oxygen. The insoluble content inthe final block was 0.19% by weight.

EXAMPLE 11 LaCl₃ Single Crystal

A 1 kg block of LaCl₃ containing 10% CeCl₃ by weight was used, thisbeing manufactured according to the invention and having an LaOClcontent of less than 0.05% by weight. This block was then used forBridgman-type growth in a graphite crucible. The nitrogen atmospherecontained approximately 50 ppm by weight of water and between 1 and 2ppm by weight of oxygen. The crystal obtained was very clear. Itsoxychloride content measured by the method of insolubles was less than0.05%. Next, a piece measuring 10×10×5 mm was cut from this crystal andits scintillation efficiency compared with a piece of NaI:TI (NaI dopedwith 600 ppm by weight of TI iodide) using the following protocol:

Photomultiplier: Hamamatsu R-1306; Reference: Nal crystal 50 mm indiameter and 50 mm in length; Integration time: 1 μs Radioactive source:¹³⁷Cs at 622 keV.

The light emission from the LaCl₃ crystal was 93% of that of the NaIreference crystal. Its energy resolution was 3.6%. The main component ofthe decay time of the scintillation was 27 nanoseconds.

EXAMPLE 12 (Comparative Example): LaCl₃ Single Crystal

1 kg of commercial LaCl₃ and CeCl₃ powders (LaOX and water contents ofexample 2) were used. The mass of CeCl₃ represented 10% of the mass ofthe blend of these two powders. They were melted in a graphite crucibleand underwent growth of the Kyropoulos (KC 01) type. The nitrogenatmosphere contained approximately 50 ppm by weight of water and between1 and 2 ppm by weight of oxygen. The crystal obtained was slightlymilky. Its content of insolubles was 0.1% by weight. Next, a 10×10×5 mmpiece was cut from this crystal and its scintillation yield comparedwith a NaI:TI using the same protocol as in the previous example. Thelight emission from the LaCl₃ crystal was 83% of that of the NaIreference crystal. Its energy resolution was 3.9%.

EXAMPLE 13 (Comparative Example): LaCl₃ Single Crystal

A crystal was produced by Bridgman-type growth in a silica crucible inaccordance with the teaching of the publication IEEE Transactions onNuclear Science: “Scintillation properties of LaCl ₃ crystals: Fast,efficient and High Energy resolution scintillators”. The mass of CeCl₃represented 10% of the mass of the mixture before growing the crystal.Next, a 10×10×5 mm piece was cut from this crystal and its scintillationyield compared with a NaI:TI piece using the same protocol as in the twoprevious examples. The light emission from the LaCl₃ crystal was 87% ofthat of the NaI reference crystal. Its energy resolution was 4.2%.

EXAMPLE 14 LaBr₃ Single Crystal

Three 1 kg blocks of LaBr₃ doped with 0.5% by weight of CeBr₃ were used,each block manufactured according to the invention and such that theLaOBr content was <0.05% by weight. This block was then used forCzochralski-type growth in a graphite crucible. The nitrogen atmospherecontained approximately 50 ppm by weight of water and between 1 and 2ppm by weight of oxygen. The crystal obtained was very clear. Theoxychloride content of this block could not be measured by the method ofinsolubles. Next, a 10×10×5 mm piece was cut from this crystal and itsscintillation yield compared with a NaI:TI piece according to thefollowing protocol:

-   -   Photomultiplier: Hamamatsu R-1306    -   Reference: NaI:TI crystal (NaI doped with 600 ppm by weight of        TI iodide) 50 mm in diameter and 50 mm in length;    -   The energy resolution of this reference crystal was 6.8% on the        ¹³⁷Cs line;    -   the measured crystals were wrapped in Teflon and coupled to the        photomultiplier (PMT) using a silicone oil (EDM fluid 200);    -   Integration time: 1 μs    -   Radioactive source: ¹³⁷Cs at 622 keV        The light emission from the LaBr₃ crystal was 147% of that of        the NaI reference crystal. Its energy resolution was 4.2%. The        main component of the decay time of the scintillation was 39        nanoseconds.

EXAMPLE 15 (Comparative Example): LaBr₃ Single Crystal

A crystal obtained by Bridgman-type growth in a silica crucibleaccording to the teachings of the publication “Applied Physics Lettersof 3 Sep. 2001 (Vol. 79, No. 10)” was compared with the previous trials.This crystal also contained 0.5% by weight of CeBr₃. Next, a 10×10×5 mmpiece was cut from this crystal and its scintillation yield comparedwith a NaI:TI piece using the same protocol as in the previous example.The crystal was slightly milky.

The light emission from the LaBr₃ crystal was 102% of that of the NaIreference crystal. The main component of the decay time of thescintillation was 38 nanoseconds.

EXAMPLE 16 LaCl₃ Single Crystal

Three 1 kg blocks of LaCl₃ doped with 5% by weight of CeCl₃ were used,each manufactured according to the invention and such that the LaOClcontent was <0.05% by weight. This block was then used for Bridgman-typegrowth in a graphite crucible. The nitrogen atmosphere containedapproximately 50 ppm by weight of water and between 1 and 2 ppm byweight of oxygen. The crystal obtained was very clear. The oxychloridecontent of this block could not be measured by the method of insolubles.It was less than 0.05% by weight. Next, a 10×10×5 mm piece was cut fromthis crystal and its scintillation yield compared with a NaI:TI pieceaccording to the following protocol:

-   -   Photomultiplier: Hamamatsu R-1306    -   Reference: NaI:TI crystal (NaI doped with 600 ppm by weight of        TI iodide) 50 mm in diameter and 50 mm in length;    -   The energy resolution of this crystal was 6.8% on the ¹³⁷CS        line;    -   the measured crystals were wrapped in Teflon and coupled to the        photomultiplier (PMT) using a silicone oil (EDM fluid 200);    -   Integration time: 1 μs    -   Radioactive source: ¹³⁷Cs at 622 keV        The light emission from the LaCl₃ crystal was 98% of that of the        NaI reference crystal. Its energy resolution was 4.6%. The main        component of the decay time of the scintillation was 28        nanoseconds.

EXAMPLE 17 Anhydrous LaCl₃

This example was as in example 1 except that the block was prepared in aplatinum crucible. The final block stuck to the crucible and was muchmore difficult to remove from the mold than in the case of the graphitecrucible coated with pyrolytic carbon.

1. A method of preparing a polycrystalline block of at least 10 g of ahalide of formula A_(c)Ln_(f)X_((3f+e)), wherein: Ln represents one ormore rare earth elements; X represents one or more halogen atomsselected from the group consisting of C1, Br and I; A represents one ormore alkali metals selected from the group consisting of K, Li, Na, Rband Cs; e, which may be zero, is less than or equal to 3f; f is greaterthan or equal to 1; and the polycrystalline block comprises less than0.1% by weight of water and less than 0.2% by weight of rare-earthoxyhalides; the method comprising: heating a mixture of at least onecompound having at least one LN—X bond and NH₄X and/or NH₄X provided ina complex to obtain a molten mass comprising the rare-earth halide ofthe formula A_(e)Ln_(f)X_((3f+e)); and cooling the molten mass to obtainthe polycrystalline block; wherein: the NH₄X and/or the NH₄X provided ina complex is present in the mixture in an amount sufficient to providethe rare-earth oxyhalides content of less than 0.2% by weight; heatingthe mixture comprises heating to a temperature that exceeds a meltingpoint of the rare-earth halide of the formula A_(e)Ln_(f)X_((3f+e)); andwhen the mixture is heated to a temperature of 300° C., a temperature ofat least 200° C. is maintained until the molten mass is obtained.
 2. Themethod as claimed in claim 1, wherein: the compound having at least oneLn—X bond is of a compound having the formulaA_(r)Ln_(s)O_(u)X_(r+3s−2u); r is from 0 to 2s; s is greater than orequal to 1; u is from 0 to s; and the compound is optionally complexedwith water or NH₄X.
 3. The method as claimed in claim 1, wherein Lnatoms of the compound having at least one Ln—X bond are linked to atomsof X, oxygen or A.
 4. The method as claimed in claim 2, wherein heatingthe mixture comprises heating without decreasing temperature until themolten mass is obtained.
 5. The method as claimed in claim 1, whereinheating the mixture comprises carrying out a temperature hold toeliminate gaseous NH₄X.
 6. The method as claimed in claim 5, whereinheating the mixture comprises heating at a rate of greater than 50°C./hour after completing the temperature hold.
 7. The method as claimedin claim 6, wherein heating the mixture comprises heating at a rate ofgreater than 100° C./hour after completing the temperature hold.
 8. Themethod as claimed in claim 7, wherein heating the mixture comprisesheating at a rate of greater than 150° C./hour after completing thetemperature hold.
 9. The method as claimed in claim 1, wherein heatingthe mixture comprises heating for less than 10 hours.
 10. The method asclaimed in claim 9, wherein heating the mixture comprises heating forless than 6 hours.
 11. The method as claimed in claim 10, whereinheating the mixture comprises heating for less than 4 hours.
 12. Themethod as claimed in claim 1, wherein: heating the mixture comprisesheating under an inert gas atmosphere; a sum of the water and contentsof the inert atmosphere is less than 200 ppm by weight.
 13. The methodas claimed in claim 12, wherein: the water content of the inertatmosphere is from 10 to 180 ppm by weight; and the oxygen content ofthe inert atmosphere is from 0.5 to 2 ppm by weight.
 14. The method asclaimed in claim 13, wherein a number of moles of NH₄X in the mixture isat least the sum of: A) a number of moles of Ln not linked to oxygen inthe at least one compound having at least one Ln—X bond; and B) threetimes a number of moles of oxygen atoms linked to Ln in the at least onecompound having at least one Ln—X bond.
 15. The method as claimed inclaim 14, wherein the number of moles of NH₄X in the mixture is at leastthe sum of: A) three times the number of moles of Ln not linked tooxygen in the at least one compound having at least one Ln—X bond; andB) five times the number of moles of oxygen atoms linked to Ln in the atleast one compound having at least one Ln—X bond.
 16. The method asclaimed in claim 1, wherein heating the mixture comprises heating in acrucible made of a material containing at least 20% carbon by weight.17. The method as claimed in claim 16, wherein the crucible comprises atleast one of carbon, glassy carbon and graphite.
 18. The method asclaimed in claim 16, wherein the crucible is coated with a layer ofpyrolytic carbon.